1. Field of the Invention
The present disclosure relates to electric power converter systems. More specifically, the disclosure relates to power converter systems suitable for interconnecting direct current output power sources (e.g., photovoltaic arrays) to a three-phase alternating current power grid.
2. Description of the Related Art
Electric power converter systems are used to transform and/or condition electrical power in a variety of applications. For example, electrical power converter systems may transform AC power from a power grid to a form suitable for a standalone application (e.g., powering an electric motor, lights, electric heater, household or commercial equipment, telecommunications equipment, computing equipment, uninterruptible power supply (hereinafter occasionally “UPS”)). Also for example, electrical power converter systems may transform power from a standalone power source such as an array of photovoltaic cells, fuel cell system, micro-turbine, or flywheel, for use in a standalone application and/or for export to, or by, a power grid.
The electrical power converter system may comprise one or more subsystems such as an DC/AC inverter, DC/DC converter, and/or AC/DC rectifier. Typically, electrical power converter systems will include additional circuitry and/or programs for controlling the various subsystems; and for performing switching, filtering, noise and transient suppression, and device protection.
In many power conversion applications, it is highly desirable to realize the maximum efficiency possible. For example, in photovoltaic applications the cost of photovoltaic arrays is still relatively high, and the physical area occupied by photovoltaic arrays may be undesirably large, particular where real estate is at a premium. Thus it is desirable to use the least number of photovoltaic cells as possible to achieve the desired power output.
In many power conversion applications power source outputs are variable or periodic in nature. For example, typical photovoltaic applications are strongly influenced by the relative movement of the sun. The time that the sun rises and/or sets, and the relative position of the sun in the sky throughout the day, determines the amount of power that the photovoltaic cells may generate. Further, the relative position of the sun throughout the year determines the amount of power the photovoltaic cells may generate, and determines the time of sunrise and sunset.
Finally, the combined cost of the power converter and the direct current generating source (e.g., photovoltaic array) are to be considered as a system. It is anticipated that the cost of photovoltaic arrays, for example, will decline. Thus, the relative cost of the power converter itself will become a more significant component of the entire system cost. Currently, the cost of power converters themselves is unnecessarily high due to the lack of generality, and high specificity of power converter designs for different applications. This lack of generality in power converter designs (i.e., highly use-specific engineering) prevents significant cost savings which may be derived from production of a generalized design which is applicable to a multiplicity of purposes.
By way of example and historical explanation, converter systems initially were purpose-built for specific applications. One early type of power converter was specifically designed for inverting direct current, constant voltage sources (e.g., batteries) to alternating current outputs (e.g., for operation of AC motors). Converters of this type are termed “inverters” and they have been in the simple form of transformers interconnecting a DC power supply with a plurality of logic control switches to generate the necessary alternating current waveform. A rectifier is another type of power converter for converting alternating current to direct current. Rectifiers have proven themselves especially useful for adapting household 110 volt alternating current to 12-volt direct current for operation of battery-powered appliances. Devices of this type have been as simple as a step-down transformer connected to a diode bridge and smoothing capacitor for full-wave rectification. U.S. Pat. No. 6,021,052 to Unger et al. entitled “DC/AC Power Converter,” issued on Feb. 1, 2000, disclosed a more sophisticated implementation of an AC to DC rectifier, including discrete components (both analog and digital) for converting direct current power to alternating current power, suitable for driving an AC load which is otherwise in series with an AC power supply. Separately, direct current to direct current (hereinafter occasionally “DC—DC”) converters have been provided for conditioning direct current power from a variable power source (e.g., a wind-driven direct current motor, photovoltaic panel or the like) for charging a battery or array of batteries.
All of the above equipment was typically purpose-built for each specific application and optimized therefore. Thus, a homeowner attempting to establish an off-the-grid power supply system may have required all three converter devices to establish a complete power system supply. By way of example, a homeowner desiring to produce power from a large photovoltaic array, may have required a DC—DC power conditioner for charging a direct current battery bank from the photovoltaic array, and a separate inverter for operating alternating current appliances from the charged, direct current battery bank so as to provide an off-the-grid power supply system. If the homeowner is located adjacent to a public power mains (three-phase power grid), the homeowner may require an AC to DC rectifier device to charge the batteries when the photovoltaic array is inoperative (e.g., at night). In order to effectively utilize a system as described above having discrete components, operator intervention is required to switch from a first mode in which the photovoltaic array charges the batteries, to a second mode in which the inverter operates an AC appliance from the charged batteries, to a third mode in which the DC batteries may be charged from the AC power grid.
In order to overcome some of the above limitations, bi-directional power converters have been developed, enabling a single power converter assembly to selectively and automatically drive an AC element (such as a motor) from a DC source (such as a battery) while utilizing the same topology to charge the DC battery when the AC element is acting as a power source (e.g., a generator or AC power main). A switching power converter of this type is disclosed by Akerson in U.S. Pat. No. 4,742,441 entitled “High-Frequency Switching Power Converter,” and issued on May 3, 1988. The switching power converter disclosed by Akerson utilizes pairs of bi-directional switches in the form of common-gated, source-to drain connected field effect transistors (FETs) driven by discrete analog and digital circuitry. By employing pulse-width modulation of the appropriately selected FETs, a sinusoidal output (or any other periodic, alternating current output) can be generated from the DC source so as to power an inductor, thereby generating the desired alternating current waveform for application to the AC element. Thus, Akerson teaches a method for converting a known DC power supply to phase and amplitude locked AC power for application to an AC power grid, and for automatically recharging the DC power supply when the DC power supply voltage falls below a predetermined level. The device disclosed by Akerson, however, is not adaptable to a DC power supply having a variable power input (such as a photovoltaic array) nor is it readily adaptable either mechanically or electrically to a different power supply (e.g., an alternating current generator motor combination). Thus, different topology and hardware is required when attempting to integrate various and diverse power supplies with, for example, an AC power grid.
UPS systems have been developed which permit power to be converted from a direct current power supply to the AC grid, and for recharging the DC power supply from the AC grid through the same apparatus. A UPS system of this type is disclosed by Unger et al. in U.S. Pat. No. 6,021,052 entitled “DC/AC Power Converter,” issued on Feb. 1, 2000. Unlike the prior art discussed above, the device disclosed by Unger et al. is capable of adapting to a variety of DC power sources by converting the variable DC input to a desired DC voltage on a DC bus. A separate system then converts the now regulated DC voltage on the DC bus into AC power for interfacing with an AC source (e.g., the AC power grid) or upon operation of a transfer switch, an AC load (such as a motor). Nevertheless, the device and topology disclosed by Unger et al. is neither adaptable nor readily modified for accepting, for example, an AC source input, nor is the electrical logic adaptable for use with a variety of different power inputs due to the heavy reliance on discrete analog and digital components for controlling the system. Thus, efficiencies of scale cannot be achieved with the device disclosed by Unger et al. because there is still a requirement to develop discrete circuits for both power handling and logical control for integrating a variety of different power sources with the AC grid.
Therefore, a need exists for a power converter topology which is readily adaptable for interfacing a variety of power sources (e.g., constant voltage DC, variable voltage DC) with a three-phase AC power grid.
A further need exists for a power converter system which addresses the above needs while being readily adaptable for optimization of power transfer characteristics when the power converter system is applied to a single input, such as a photovoltaic array.